c - 为什么 pthread_cond_signal 不起作用？

Question

我目前正在学习 POSIX 线程 (pthread)。

我现在创建了一个简单的程序，它将共享值增加 7 直到超过 10000，然后它应该向下一个线程发出条件信号，将其减少 3 直到低于 1000。最后它应该将结果除以 2 和 main应该输出结果。

My code :

pthread_t threads[3];
pthread_cond_t cond_a, cond_b;
pthread_mutex_t mutex;

int counter;

void * worker_one();
void * worker_two();
void * worker_three();

int main(int argv, const char ** argc) {
    counter = 0;

    pthread_cond_init(&cond_a, NULL);
    pthread_cond_init(&cond_b, NULL);
    pthread_mutex_init(&mutex, NULL);

    pthread_create(&threads[0], NULL, worker_one, NULL);
    pthread_create(&threads[1], NULL, worker_two, NULL);
    pthread_create(&threads[2], NULL, worker_three, NULL);

    pthread_join(threads[0], NULL);
    pthread_join(threads[1], NULL);
    pthread_join(threads[2], NULL);

    printf("Value started at %d and ends with %d.\n", 0, counter);

    return 0;
}

void * worker_one() {
    printf("Worker one started.\n");

    pthread_mutex_lock(&mutex);

    printf("Worker one starting work.\n");
    while (counter < 10000) {
        counter += 7;
    }

    pthread_cond_signal(&cond_a);

    printf("Worker one finished work with: %d.\n", counter);

    pthread_mutex_unlock(&mutex);
    pthread_exit(NULL);
}

void * worker_two() {
    printf("Worker two started.\n");

    pthread_mutex_lock(&mutex);
    pthread_cond_wait(&cond_a, &mutex);

    printf("Worker two starting work.\n");
    while (counter > 1000)
        counter -= 3;

    printf("Worker two finished work with: %d.\n", counter);

    pthread_cond_signal(&cond_b);
    pthread_mutex_unlock(&mutex);

    sleep(1);

    pthread_exit(NULL);
}

void * worker_three() {
    printf("Worker three started.\n");

    pthread_mutex_lock(&mutex);
    pthread_cond_wait(&cond_b, &mutex);

    printf("Worker three starting work.\n");

    counter /= 2;

    printf("Worker three finished work with: %d.\n", counter);

    pthread_mutex_unlock(&mutex);
    pthread_exit(NULL);
}

出于某种原因，整个执行都卡在第一个线程周围。信号也被触发，但线程二没有反应。

谁能告诉我我做错了什么？

Answer 1

我在这里回答过类似的问题:pthread condition variables on Linux, odd behaviour .

问题是你甚至在测试你想要等待的条件为真之前就等待了。发生的情况是线程 1 在线程 2 等待之前发出信号，因此信号丢失，线程 2 将永远等待。

为了避免这种情况，首先测试你想要等待的是什么，然后只有当它不在这里时才等待。

编辑:好的，这是一个可能的解决方案，只有一个互斥锁和一个条件(未经测试)

线程 1:

pthread_mutex_lock(&mutex); 
while(thread_1_should_work == false){ // wait until the condition is satisfied
  pthread_cond_wait(&cond, &mutex); 
}

//at this point, we owe the mutex and we know thread_1_should_work is true; 

// do work 

thread_1_shoudl_work = false; 
thread_2_should_work = true; 

pthread_cond_broadcast(&cond); //wake up any waiting thread (if it's not their turn, they'll call wait again)
pthread_mutex_unlock(&mutex); 

...等等。